Choosy females and indiscriminate males: mate choice in mixed populations of sexual and hybridogenetic water frogs (Rana lessonae, Rana esculenta)

For several decades, behavioral ecologists have studied the effects of the environment on the behavior of individuals; but only fairly recently they h ave started to ask the reverse question: how do the behavioral strategies o f individuals affect the composition and dynamics of populations and commun ities? Although intuitively obvious, this feedback From individual to highe r levels is difficult to demonstrate, except in systems with exceptionally fast and marked responses of the populations to the behavior of its members . Such a system exists in sperm-dependent species. In European water frogs, for instance, successful reproduction of a hybrid species (R. esculenta, g enotype LR) requires mating with one of its parental species (R. lessonae, genotype LL), except in the rare cases where hybrids are triploid. The sexu al host LL, however, should avoid matings with the sexual parasite LR, beca use the resulting LR offspring will eliminate the L genome from their. germ line. In this study we investigate how this conflict is solved. Since wate r frog hybrids come in both sexes, rather than as females only like in othe r sperm-dependent systems, we performed the tests with both females and mal es. One individual was given a choice between two individuals of the opposi te sex, one an LL and the other an LR. In both species, females showed the predicted preference for LL males, whereas males did not discriminate betwe en LL and LR females. On the individual level, we interpret the sex differe nce in choosiness by the lower costs from mating with the wrong species (LR ) and the higher benefits from mating with large individuals in males than in females. In "normal" species, male preference for large (i.e. more fecun d) females is advantageous, but in this system such a choice can result in mating with the larger LR females. With respect to the structure and dynami cs of mixed populations, we discuss that the observed female preference is consistent with the higher mating success of LL males found in nature. Henc e, mate female choice is a strong candidate for a mechanism promoting coexi stence of the sperm-dependent hybrid and its sexual host. This confirms pre dictions from previous theoretical models.